Field of the Invention and Related Statement
The present invention concerns a tread rubber composition having excellent performance on ice and suitable to a studless tire, as well as a tire having a tread manufactured by using the rubber composition.
A spiked tire exhibits an excellent gripping force frozen road surface. Since the spiked tire has a large digging friction force, among hysteresis loss friction, adhesion friction and digging friction related to the gripping force between a tread rubber and a road surface, it exhibits an excellent gripping force on the frozen road surface.
However, use of the spiked tire has gradually been restricted legally as a countermeasure for public pollution caused by powdery dusts in recent years, and a studless tire with no spikes has been rapidly popularized. In some studless tires, the digging friction force is improved, for example, by incorporating a blowing agent or organic fibers therein thus increasing the unevenness on the surface of the tread. However, since the studless tire has lower digging friction force as compared with the spiked tire, its gripping force is still inferior to that of the spiked tire. In particular, if the road surface is ground flat by idle rotation of a tire upon starting on the frozen road surface, the gripping force of the studless tire is even more deteriorated, so that a further improvement has been demanded.
On a road surface with extremely low friction coefficient, such as a frozen road surface, the hysteresis loss friction is extremely small but adhesion friction (tract) also contributes to improvement of the gripping force, in addition to the digging friction. Therefore, it has been devised to increase the gripping force on the frozen road surface for the studless tire by increasing the other of the frictions contributing to the gripping force, that is, the tract.
As a method of improving the gripping force on the frozen road surface, there is a method of improving the tract by eliminating hydroplanes caused by thawing of the frozen road surface due to friction upon starting and braking, thereby increasing the area of contact between a tread rubber and an icy surface, or by making the rubber material softer to increase the area of contact with the road surface.
Increased unevenness on the tread surface can increase the digging friction, as well as can take up peripheral water into concave portions to eliminate hydroplanes. However, increase of the concave portions for the elimination of hydroplanning leads to a decrease in the area of contact with the icy surface, which is contrary to the improvement of the gripping force based on the increase in the area of contact, so that the effect is limited.
On the other hand, U.S. Pat. No. 4,522,970 discloses that a tread composition incorporating kaolinite clay and 3,3'-bis(trimethoxysilyl propyl)polysulfide in a certain rubber ingredient can improve wet skid resistance. However, tread rubbers for use in a studless tire in disclosed examples have an insufficient performance on ice. In addition, blending of the clay generally deteriorates the reinforcing performance of the rubber, and the rubber ingredient (butadiene and SBR) used for the tire in the disclosed examples shows large amounts of wear, which is not suitable for a tread rubber.